1. Field of the Invention
The present invention relates to a lens dyeing method and a lens dyeing apparatus, and more particularly, to a lens dyeing method for plastic lenses for use in eye glasses in such a manner that a color density gradient occurs on the plastic lenses and that the boundary between the dyed portion and the non-dyed portion is vague, as well as a lens dyeing apparatus for use in carrying out such a lens dyeing method.
2. Description of the Related Art:
Recently, plastic lenses have been the main type of lens used in eye glasses. Plastic lenses can be readily dyed and therefore employed to make colorful fashionable lenses. Particularly, lenses, on which dyeing is performed in such a manner that the gradient of change in the color density is continuous, can be employed as lenses of highly fashionable eye glasses and thus possess a higher added value. Such lenses are called gradient lenses, gradated lenses or half-colored lenses. Conventionally, dyeing of such gradient lenses is conducted in the manufacturing plants or eye glass shops using a dyeing apparatus which includes a dyeing bath containing a dyeing solution, a device for adjusting the temperature of the dyeing bath and a device for holding and dipping the lenses into the dyeing bath.
In the gradated dyeing method for manufacturing gradient lenses, when the color of a designated area of the lens surface is to be dyed in gradations, a continuous color density gradient is imparted to the boundary portion between the dyed area and the non-dyed area on the lens surface by dipping the designated portion of the lens into the dyeing solution while changing the dipping time of the designated portion and gradually changing the designated portion which is dipped. In a practical operation, the designated portion of the lens is dipped in the dyeing solution contained in the dyeing bath with a motion comprised of a combination of an up and down reciprocating motion of the lens having a small amplitude from a lowest position to a highest position and a short cycle time superimposed on an up and down reciprocating motion of the lens having a large amplitude from a lowest position to a highest position and a long period, as is disclosed in Japanese Patent Laid-Open No. 97064/1978. FIG. 3 shows how the lens position changes by the combination of such superimposed motions. In FIG. 3, a reference numeral 31 denotes a trace of the position of the lens, e.g., the trace of the lowest end portion of the lens caused by the change in the position thereof. In the graph of FIG. 3, the axis of abscissa represents the time, and the axis of ordinate represents the position of the lowest end portion of the lens (the depth of the lowest end portion of the lens from the surface of the dyeing solution contained in the dyeing bath). A reference numeral 32 designates the position of the surface of the dyeing solution in the dyeing bath.
FIG. 4 is a graph showing another method of changing the position of the lens dipped into the dyeing solution, which is disclosed in Japanese Patent Laid-Open No. 43427/1983. In this dipping method, the lens is periodically moved up and down in a vertical line, and as features of the up and down motion, a period of a cycle of the motion is relatively short, and a lowest position of the lens for each cycle gradually rises in the dying solution.
The dyeing method of manufacturing gradient lenses is characterized by the dyeing of the boundary portion between the dyed area and the non-dyed area in the lens such that a clear boundary line of color density is not generated on this boundary portion. As to whether or not the boundary line is formed on the boundary portion depends on the dipping time of the boundary portion of the lens and the controlling method of the up and down reciprocating motion having a large amplitude from a lowest position to a highest position and a long period which is associated with the changes in the position of the lens during the dipping.
In the above dyeing methods disclosed in Japanese Patent Laid-Open Nos. 97064/1978 and 43427/1983, only the dipping time of the lens is adjusted and no proposal is made on the controlling method of the up and down reciprocating motion having a large amplitude. More specifically, the up and down reciprocating motion having a large amplitude draws, for example, a sine curve which always changes at fixed characteristics, as shown in FIG. 5. This means that the lowest depth to which the lens is dipped into the dyeing solution is fixed, and that the lens is always dipped to the fixed lowest position in the dyeing solution. In consequence, if the conventional dyeing method is applied to the dyeing of lenses using a dyeing solution which is capable of dyeing at a high speed or having a high temperature (e.g., 90.degree. C.), which results in a dyeing having high dyeing capability and in which the dyeing density is not proportional to the dipping time, and dyeing of the surface of the lens proceeds at a high speed, making it very difficult to dye the boundary portion of the lens in such a manner that the gradient changes in color density is continuous
Furthermore, once plastic lenses possessing hydrophobic property are dipped into the dyeing solution, a film of water is formed on the entire surface of the lens due to the surface tension of the dyeing solution, and the lens surface is thereby made hydrophilic to some extent. In the case of lenses subjected to a hard coating process which employs a coating mainly made of organic silicon and which increases the water repellency of the lens surface, dipping of the lens into the dyeing solution does not itself create the hydrophilic property of the lens but only intensifies the hydrophobic property thereof, increasing the ease with which the boundary line is formed on the boundary portion.
Furthermore, the temperature of the dyeing solution contained in the stain dyeing bath is generally about 70.degree. to 90.degree. C., so the dipped portion of the lens is almost the same temperature whereas the non-dipped portion of the lens is exposed to air, thereby generating a difference in temperature between the dipped and non-dipped portions. As a result, a boundary line may be formed on the boundary portion of the lens and it may become difficult to impart a continuous color density gradient to that boundary portion.